The invention relates to a matrix which is suitable for use in an injection molding process or a compression molding process for the manufacture of optically readable information carriers which comprise an optically readable information track.
The optically readable information carriers are synthetic resin discs manufactured, for example, from polymethylmethacrylate (PMMA) or polycarbonate. The information track, which is provided in the surface of the disc, has a crenellated profile of information areas situated alternately at a higher level and at a lower level. The areas have small dimensions, for example, longitudinal dimensions varying from 0.3 to 3 .mu.m. The difference in height between the areas is 0.1-0.2 .mu.m. On the side of the information track the disc is covered by a reflection layer, for example a layer of Ag or Al. The track is read in reflection by means of a laser light beam. Reading is based on differences in phase in the reflected light dependent on whether an area situated at a higher level or an area situated at a lower level is irradiated.
The discs are manufactured by using an injection molding process or a compression molding process in which a liquid synthetic resin composition is injected into a mold under pressure. In an injection molding pressure the liquid synthetic resin is injected in a mold which is already closed; in a compression molding process the mold is closed during or immediately after injection. One or two matrices are provided in the mold. The matrix has an information track which is the negative of the desired information track of the final product.
The known matrix used for the manufacture of optically readable information carriers is a nickel matrix. The manufacture thereof there is started from a master disc. The master is a glass plate which is provided on one side with a photoresist layer in which an information track is provided by exposure to light and development. On the side provided by the photoresist layer, comprising the information track, the master is provided with an electrolessly, evaporated or sputtered metal layer, for example, an Ag layer. On this metal layer an Ni peel is grown by electrodeposition forming the so-called father disc. The master is removed and Ni copies, the so-called mother discs, are made from the resulting father disc by electrodeposition.
From the mother discs Ni copies are made, the so-called dies, which are used in the above-mentioned injection molding process or compression molding process.
The disadvantage of the above-mentioned method is that in the manufacture of the father disc the master disc is lost. Hence only one Ni father disc can be manufactured from the master. This is a serious disadvantage in view of the high cost-price of the master. The above-described process for the manufacture of an Ni matrix is also time-consuming and comprises a series of process steps each of which gives rise to the occurrence of defects. Moreover, each Ni peel manufactured by electrodeposition must be polished. Loss of quality and/or a comparatively high reject percentage is the result.